As means for imparting corrosion resistance to metal materials, it has heretofore been widely attempted to coat the surfaces of metals with a resin layer. As coating methods used for this technology, further, there have been known a method of coating the surfaces of a metal with a solvent in which there has been dispersed a thermosetting resin such as epoxy resin, phenol resin, acrylic resin or polyester resin, and a method of sticking a film that has been formed in advance such as polyester film, olefin film or polyamide film onto a metal substrate via an adhesive such as isocyanate adhesive, epoxy adhesive or phenol adhesive.
It has further been widely known to utilize the heat-melting property of the thermoplastic resin for sticking the metal substrate and the thermoplastic resin together. For this purpose, there have been known a method of sticking a film that has been formed in advance using a thermoplastic polyester resin or the like onto a metal plate by heat-adhesion, and a method of sticking a thin molten film of a thermoplastic polyester resin that is extruded onto a metal plate.
The latter method of sticking onto the metal plate based on the extrusion lamination is capable of executing the treatment at a very high speed offering an advantage of decreasing the amount of work involved in the formation of films and decreasing the cost.
As the extrusion lamination method, there has generally been employed a T-die method using an extruder and a T-die. The T-die method, however, involves unstable flow in the extruder and in the die and, besides, an air gap exists to some extent between the T-die and the metal plate. When a generally employed polyester is used, therefore, the film oscillates and pulsates making it difficult to form a coating on the metal plate maintaining stability, uniform film thickness and good adhesion. These phenomena occurs particularly when the resin is taken up at an increased speed, making it very difficult to laminate the polyester resin at high speeds.
In order to solve the above-mentioned problems, there have been proposed various methods such as the one taught in JP-A-10-86308 filed by the present applicant, which discloses a laminate having a ratio of melt viscosities at a temperature of melt extrusion (melt viscosity η12.2 at a temperature of extruding the polyester and at a preceding stage rate of 12.2 sec−1/melt viscosity η1216 at the temperature of extruding the polyester and at a shearing rate of 1216 sec−1) of not smaller than 2.0, and a melt viscosity η1216 of not smaller than 500 poises, wherein the resin layer is melt-extruded and is, then, quickly cooled, the polyester resin used therein containing an ethylene oxide adduct of bisphenol or a trifunctional or more functional polybasic acid and a polyhydric alcohol, the laminate exhibiting a suppressed draw resonance phenomenon and forming a coating having excellent homogeneity and adhesiveness relying upon a high-speed lamination.
JP-A-2001-72747 discloses a polyester having a melting point of not lower than 220° C. comprising chiefly an ethylene terephthalate containing a compound that has three or four ester bond-forming functional groups in an amount of 0.1 to 2.0 mol %, the polyester having a die swell ratio of not smaller than 1.3 as measured at a temperature of the melting point of the polyester plus 40° C. and exhibiting excellent extrusion workability.